Studying of Surfactant Excess Separation from Non-aqueous Quantum Dots Solution on its Monolayer Formation Process

Controlled purification of non-aqueous solution of CdSe/CdS/ZnS quantum dots (QD) (dispersed in toluene or chloroform) from an excess of surfactant (oleic acid) was studied. The separation of unbound oleic acid from QD solution was carried out by using high-density polyethylene (HDPE) membrane. The secondary solution with a separated surfactant as well as initial QD solution was spread over the water surface to obtain a monolayer with its compression isotherms studied using Langmuir-Blodgett (LB) technique. This approach allowed us to determine the concentration of surfactant at a level of 10−5 mol/l or less for CdSe/CdS/ZnS QD. For better understanding of the influence of purification on the properties of QD, the monolayer of QD was transferred onto solid substrate and its morphology was studied. Atomic force microscopy showed that removal of oleic acid excess results in a more uniform and dense packing of quantum dots.